Amplitude-modulation (AM) to AM (AM-AM) distortion is a third order intermodulation distortion (IMD) effect by which the magnitude of the output signal of a system is compressed or expanded, referred to as compansion or the state of being companded, in response to changes in current amplitude in a digital power amplifier (DPA), or in radio-frequency (RF) power amplifiers in general. Highly linear amplifiers exhibit the least distortion and, as such, are preferred when signal quality has precedence over cost. However, such highly linear amplifiers operate at high power levels and are relatively large and expensive. And, while such linear amplifiers offer the best performance, they are typically not used in wireless applications where cost is a factor.
To strike a balance between efficiency, linearity and cost, a DPA may be designed to exhibit known nonlinearity. Predistortion may then be used to compensate for this nonlinearity, resulting in a linearization of the output of the DPA. That is, if the DPA's distortion characteristics are known in advance, an inverse function, referred to herein as a predistortion function, can be applied to the input data prior to reaching the DPA. The DPA distortion is essentially cancelled by the predistortion resulting in an amplified output signal having substantially reduced distortion.
Digital predistortion is most often carried out through a lookup table (LUT) that associates predistorted output data values with input data values. That is, entries in the LUT are addressed by the samples of the input data. The LUT is typically populated by way of calibration procedures designed to detect AM-AM distortion. One limitation of LUT-based predistortion in digital transmitters using a DPA is that data samples must be converted and transmitted at a very high rate. Thus, a predistorter must look up and retrieve output values very quickly. Another drawback is the LUT's inherent quantization, even when the LUT is interpolated. This is especially problematic at low output current levels, i.e., when fewer current cells in the DPA are conducting. Activating additional current cells in the DPA responsive to an increasing number of bits in the data represents a higher degree of current change from step to step than at higher power levels. As such, quantization effects have a greater impact at lower power levels than at higher power levels.
Given the state of the current art, the need is apparent for a high dynamic range predistortion technique that results in smoother transitions between power levels across the entire range of input data while realizing an arbitrarily nonlinear predistortion function.